Paper machine stock contains in addition to water and fibers also necessary additives and depending on the treatment of the stock and its grade, air and other gases. The entrained air may disturb the web forming process and affect the quality of the finished product. In general, the paper stock is, therefore, deaerated before it is supplied to the headbox of the paper machine. The most common deaerating method is vacuum deaeration by allowing the paper stock to expand in a tank connected to a condenser and a vacuum pump. In the tank, a reduced pressure is maintained which approximately corresponds to the boiling pressure of water at the temperature prevailing within the tank. The temperature in the tank, in turn, depends on the cooling water temperature of the condenser. In geographical areas with high ambient water temperatures, such as 20.degree.-26.degree. C., this represents a rather severe restriction on the operation of the system. Methods and apparatus for accomplishing deaeration of papermaking stock are disclosed, for instance, in U.S. Pat. Nos. 3,206,917 and 4,443,232.
It is further known that the water vapor pressure above concentrated water-salt solutions of, for example, calcium chloride, lithium chloride, lithium bromide or the like and/or mixtures thereof is lower than that of pure water at a corresponding temperature and that these solutions can therefore be used as water-absorption liquids.
The present invention utilizes the low vapor pressure above a concentrated salt solution for allowing a saturated vapor to be absorbed in a salt solution having a higher temperature than the absorbed vapor.
According to one aspect of the invention, paper machine stock is deaerated by causing it to expand in a flash tank and condensing the water vapor produced thereby into a water-absorbing liquid on a cooled heat exchange surface. Preferably, the flash cooled paper machine stock is used as a cooling medium in the condenser.
Accordingly, one advantage of the present system over the conventional system is that the cooled paper stock can be used as cooling medium due to the "heat pumping" effect when the water vapor is absorbed into the hygroscopic solution. Normally, cooling water having a temperature of 25.degree. C. would be sufficient for the flash cooling of paper stock having a temperature of 44.degree. C. (44.degree.-60.degree. C.), but the heat generated would be lost to a lower level by heating the 25.degree. C. cooling water to, e.g. 30.degree. C.
If the present system is used for producing extremely low pressures a typical example would be that the suspension is flash cooled from 30.degree. C. to 25.degree. and the vapor is condensed into the hygroscopic solution at a temperature of 45.degree. C. and the heat of condensation is removed by constantly cooling the condenser using cooling water of, e.g. 25.degree. C. being heated to 30.degree. C. This would, of course, be impossible to perform with a conventional system using normal surface condensers.
The invention will also permit the complete deaeration of suspensions of solids in a liquid at a vapor pressure much lower than that present when normal cooling water is used. Additionally, the heat which is released when the suspension is flash cooled is preferably transferred back to the suspension when it is used as a cooling medium in the condenser, thus recovering substantially all of the heat.
The absorption liquid diluted by the vapor condensed into it is thereafter regenerated in an evaporator and recirculated back to the condenser.